Voltage controlled oscillator circuit, device, and method

ABSTRACT

A voltage-controlled oscillator (VCO) includes a power supply node configured to have a power supply voltage. A reference node is configured to have a first reference voltage. A transformer-coupled band-pass filter (BPF) is coupled to a cross-coupled pair of transistors. The cross-coupled pair of transistors and the transformer-coupled band-pass filter are positioned between the power supply node and the reference node.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/691,928 filed Jun. 29, 2018, which is incorporated herein by reference in its entirety.

BACKGROUND

A voltage controlled oscillator (VCO) is an oscillator with an output signal whose output can be varied over a range, which is controlled by an input voltage. The output frequency of the output signal of the oscillator is directly related to the input voltage. The oscillation frequency varies from a few hertz to hundreds of gigahertz (GHz). By varying the input voltage, the output frequency of the output signal is adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A-1B are schematic diagrams of a voltage controlled oscillator (VCO), in accordance with some embodiments.

FIGS. 2A-2B are a schematic diagram and graph of the phase noise performance of a VCO, in accordance with some embodiments.

FIG. 3 is a graph of the transmission coefficient of a VCO, in accordance with some embodiments.

FIG. 4 is a schematic diagram of a transformer of a transformer-coupled bandpass filter (BPF), in accordance with some embodiments.

FIGS. 5A-5B are schematic diagrams of a transformer-coupled BPF, in accordance with some embodiments.

FIGS. 6A-6B is a schematic diagram of a transformer-coupled BPF and graph of the isolation of the transformer-coupled BPF, in accordance with some embodiments.

FIG. 7 is a schematic diagram of a transformer-coupled BPF, in accordance with some embodiments.

FIGS. 8A-8B are graphs of the noise suppression of a VCO, in accordance with some embodiments.

FIGS. 9A-9B are graphs of the measured oscillation frequency and measure phase noise of a VCO, in accordance with some embodiments.

FIG. 10 is a flowchart of a method of generating an oscillation signal, in accordance with one or more embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components, values, operations, materials, arrangements, or the like, are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Other components, values, operations, materials, arrangements, or the like, are contemplated. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Voltage controlled oscillators (VCOs) are used as part of a phase locked loop (PLL) to synchronize the VCO frequency to a reference frequency. The quality of the output of a VCO is compromised when short term random frequency signal fluctuations, called phase noise, appear at the output. Phase noise introduces second order and third order harmonic spectral components that alter the output of a VCO. For a VCO to be operable with certain millimeter wave applications (mmWave) (30 GHz-300 GHz), the VCO needs to control the amount of phase noise that appears at the VCO output.

FIG. 1A is a schematic diagram of a VCO 100, in accordance with some embodiments. VCO 100 has two components: a resonator 102 and a transformer-coupled bandpass filter (BPF) 106. The resonator 102 is used for frequency detection and replication for the VCO 100. The resonator 102 includes two bias-tee (bias-T) circuits 104 and 108 that provide DC voltage or DC current in order to bias the resonator 102. The bias-T circuit 104 includes an inductor L1, a node 114, and a capacitive device C1. The inductor L1 is connected to the node 114 at one end and a DC voltage power source (VBUF) at the other end. The capacitive device C1 is connected to the node 114 at one end and a resistor R1 at the other end. The resistor R1 is connected to a ground source at the other end. The bias-T circuit 108 includes an inductor L2, a node 116, and a capacitive device C2. The inductor L2 is connected to the node 116 at one end and a DC voltage power source (VBUF) at the other end. The capacitive device C1 is connected to the node 116 at one end and a resistor R2 at the other end. The resistor R2 is connected to a ground source at the other end. The bias-T circuits 104 and 108 are used to provide a fixed DC voltage to transistors M3 and M4.

Transistor M3 is connected at a drain terminal to the node 114 and is connected at a source terminal to a ground source. A gate of transistor M3 is connected to a node 118. A variable capacitive device C6 is connected to the node 118 at one end and to an electrical node 120 at the other end. Transistor M4 is connected at a drain terminal to the node 116 and is connected at a source terminal to a ground source. The gate of transistor M4 is connected to a node 122. A variable capacitive device C3 is connected to the node 122 at one end and to the node 120 at the other end. The node 120 is connected to a first reference voltage V_(ctrl1).

A drain terminal of transistor M1 is connected to a node 124 and a source terminal is connected to a ground voltage. A drain terminal of transistor M2 is connected to a node 126, and a source terminal is connected to a ground voltage. A gate of transistor M1 is connected to a node 128, and a gate of transistor M2 is connected to a node 130. A variable capacitive device C5 is connected to the node 128 at one end and to a node 132 at the other end. A variable capacitive device C4 is connected to the node 130 at one end and to the node 132 at the other end. The node 132 is connected to a second reference voltage source (V_(ctrl2)). The transistors M1 and M2 are configured to form a cross-coupled pair of transistors.

The transformer-coupled BPF 106 includes a pair of coupled-transformers T1 and T2, and a pair of coupled capacitive devices C_(c1) and C_(c2). Transformer T1 includes a primary winding W1, a secondary winding W2, and a core CR1. The transformer T2 includes a primary winding W3, a secondary winding W4, and a core CR2. The transformer T1 and T2 are configured to operate together as a 1:2 transformer in this embodiment, however, in other embodiments the transformers T1 and T2 have different transformer implementations depending on power considerations of the VCO. The transformer T1 produces a phase difference of 180 degrees as indicated by the polarity dots 144 for transformer T1 while the phase produced by the transformer device T2 is −180 degrees as indicated by the polarity dots 146. The primary winding W1 is connected to node 134 at one end and to a power supply source (VDD) at the other end and the primary winding W3 is connected to node 136 at one end and to the power supply source (VDD) at the other end. The capacitive device C_(c1) is connected between nodes 134 and 136. The secondary winding W2 is coupled to node 138 at one end and to voltage source (V_(G)) at the other end and the secondary winding W4 is connected to node 140 at one end and to voltage source (V_(G)) at the other end. The capacitive device C_(c2) is connected between nodes 138 and 140. The coupling of the pair of coupled capacitive devices C_(c1) and C_(c2) with the transformer T1 and T2 form a BPF.

The connection of node 134 to node 124, node 136 to node 130, node 138 to node 128, and node 140 to node 126 connects resonator component 102 to the transformer-coupled BPF 106.

FIG. 1B is a schematic diagram of a VCO 110, in accordance with an embodiment. VCO 110 comprises the resonator component 102. VCO 110 differs from VCO 100 in the configuration of a transformer-coupled BPF 112 in connection with VCO 110. The transformer-coupled BPF 112 is similar to the transformer-coupled BPF 106 except for the connection of the pair of coupled capacitive devices C_(c1) and C_(c2). In VCO 110, the capacitive device C_(c1) is connected between node 134 and node 138, and the capacitive device C_(c2) is connected between node 136 and node 140. Both ends of the capacitive devices C_(c1) and C_(c2) of VCO 110 are connected to the terminals of the same transformers T1 and T2, which is opposite the connection of the capacitive devices C_(c1) and C_(c2) for VCO 100.

The cross-coupled pair of transistors M1 and M2 are configured to produce negative resistance to compensate for signal loss from the transformer-coupled BPF 106. The transistors M1 and M2 operate in the saturation region for current stability to reduce 1/f (also known as flicker noise) caused by charge trapping and releasing in the transistors M1 and M2; however, removing flicker noise does not reduce phase noise. Flicker noise is not a predominant factor at higher frequency ranges, such as 1 GHz or higher, where phase noise is more predominant. To reduce phase noise, the transformer-coupled BPFs 106 and 112 use the pair of coupled capacitive devices C_(c1) and C_(c2) to filter and reduce phase noise contributed by higher harmonics, such as 2f₀, 3f₀, or higher, where f₀ is a lower cutoff frequency of the transformer-coupled BPF 106 or 112 described herein. The capacitive devices C_(c1) and C_(c2) are each coupled to the pair of coupled-transformers T1 and T2 to form the transformer-coupled BPF 106 or 112. The frequency response of the transformer-coupled BPF 106 or 112 includes an additional transmission zero, defined as the frequency where the frequency response produces a nearly zero value. Capacitive devices, such as C_(c1) and C_(c2), increases the number of transmission zeros in a frequency response of a system due to filtering. The transmission zero for transformer-coupled BPFs 106 or 112 occurs at twice the lower cutoff frequency (2f₀), where the lower cutoff frequency (f₀) is the lowest corner frequency of the transformer-coupled BPF 106 or 112. Moreover, the pair of coupled capacitive devices C_(c1) and C_(c2) are configured so that the transformer-coupled BPF includes a bandpass range outside the frequency range of the phase noise contributed by the 2^(nd) and 3^(rd) harmonics. In at least some embodiments, the transformer-coupled BPF 106 or 112 reduces phase noise by 14 dB or more.

In some embodiments, the transistors M1-M4 are bipolar transistors, field effect transistors (FETs), or the like. In some embodiments, the transistors M1-M4 are metal-oxide semiconductor field-effect transistors (MOSFETs), such as CMOS, NMOS, PMOS, or the like. In some embodiments, the transistors M1-M4 are different types of transistors. In some embodiments, the ground source described is external to the VCO or internal ground to the VCO. In some embodiments, the variable capacitive devices C3-C6 are varactor structures or the like that allow capacitance to change based on voltage or current.

FIG. 2A is a schematic diagram of a feedback circuit of a VCO 202, in accordance with an embodiment. The VCO 202 includes a first BPF 204 having a gain ml that receives as input a signal D1. The first BPF 204 produces an output 206 that is a result of the multiplication of the gain m₁ and D1. A second BPF 208, having a gain m₂, receives as input a signal D2. The second BPF 208 produces an output signal 210 that is a result of the multiplication of a gain m₂ and D2. An adder 212 receives as input the signal 206 and the signal D2 and outputs a signal G2. An adder 214 receives as input the signal 210 and the signal D1 and outputs a signal G1. A conductance device 216 receives as input the signal G1, and outputs the signal D1. A conductance device 218 receives as input the signal G2, and outputs the signal D2. A feedback arm 220 is configured to include the conductance device 216 and adder 214 so the signal D1 produces consistent results. A feedback arm 222 is configured to include the conductance device 218 and adder 212 so the signal D2 produces consistent results.

In some embodiments, the gains m₁ and m₂ are related to the turn ratios of each of the transformers of the pair of coupled-transformers as described herein (e.g. “T1” and “T2”). In some embodiments, the gains m₁ and m₂ are the same or different. In some embodiments, the conductance devices 216 and 218 are the conductance of each of the transformers of the pair of coupled-transformers T1 and T2 of FIGS. 1A and 1B. In some embodiments, the conductance devices 216 and 218 include circuits that are external to the VCO 202.

The BPFs 204 and 208 increase the oscillation amplitude by applying gains m₁ and m₂ to signals D1 and D2 without requiring additional DC voltage. In this case, the signals D1 and D2 are substantially similar and the gains m₁ and m₂ are 2, and the transformers used by the BPFs 204 and 206 have turn ratios of 2. Moreover, all parasitic losses are negligible. Using these parameters, the signal power of G1 (P_(G1)) is approximately three times the signal power of D1 (P_(D1)) based on the feedback circuit of the VCO 202 and the following equation:

P _(D1) +m ₂ P _(D2) =P _(G1)

-   -   If P_(D1)=P_(D2) and m₁=m₂=2     -   P_(G1)=3P_(D1).

The noise power does not increase due to the phase noise suppression of the BPFs 204 and 208. The VCO 202 produces a phase reduction of approximately 10 log(⅓)≈−4.8 dB.

In some embodiments, the turn ratio is higher than 2 and signals D1 and D2 are not similar.

FIG. 2B is a graph of the phase noise performance 224 of the VCO 202, in accordance with some embodiments. Plot 224 is a graph of the phase noise improvement in decibels (dB) versus turn ratio. As the turn ratio increases, phase noise improvement increases, i.e., dB levels fall, as depicted in plot 224.

FIG. 3 is a graph 300 of the transmission coefficient of a VCO, in accordance with some embodiments. The transmission coefficient of the VCO indicates the amplitude and power outputted by the VCO. The plot 300 includes a curve 302 and a curve 304. The curve 302 includes the transmission coefficient versus frequency of a pair of coupled-transformers according to an approach not having a transformer-coupled BPF as described herein, and the curve 304 includes the transmission coefficient versus frequency of a transformer-coupled BPF (e.g. “106” or “112”) of FIGS. 1A or 1B. In this case, the transformer-coupled BPF has a lower cutoff frequency (f₀) of approximately 28 GHz. The curve 304 includes a transmission zero at approximately 56 GHz or (2f₀) with more than a 14 dB drop relative to the curve 302 at the same point. At 84 GHz, the curve 304 includes a drop of approximately 14 dB relative to the curve 302. The pair of coupled capacitors filters out the higher frequency components of the phase noise.

FIG. 4 is a schematic diagram of a pair of coupled-transformers 400 used in a transformer-coupled BPF, in accordance with some embodiments. The pair of coupled-transformers 400 is configured in a two stacked metal layer arrangement. The pair of coupled-transformers includes a number of conductive structures 406, 412, and 414 formed on a metal layer. Also, the pair of coupled-transformers 400 include a conductive structure 418 formed on the metal layer having a first terminal 434 at one end of the coupled-transformer 400 where two extending portions 438 extend from the first terminal 434 to two terminals 422 and 426 at the opposite end. The extending portions 438 are configured to be the primary windings W1 and W3 shown in FIGS. 1A-1B.

The conductive structure 412 includes a terminal 424 at one end and two oppositely extending portions 436 at the opposite end of the conductive structure 412. The two oppositely extending portions 436 extend from the terminal 424. The conductive structure 406 includes a first end that is connected to one of the extending portions 436 near the first terminal 434. The conductive structure 406 includes a terminal 420 positioned at an opposite end from the first end. The conductive structure 414 includes a first end that is connected to another of the extending portions 436 of the conductive structure 412 near the first terminal 434. Also, the conductive structure 414 includes a terminal 428 positioned at an opposite end from the first end. Conductive structures 406 and 414 connect with the extending portions 436 of the conductive structure 412 below the first terminal 434.

Via structures 430 and 432 are configured to provide the connections necessary to form the pair of coupled-transformers 400. A portion of the conductive structures 406, 412, 414, and 418 are positioned above a first substrate 404. The via structures 432 are configured to make connections between the conductive structures 406 and 412 to form the secondary winding W2 of FIGS. 1A-1B and between conductive structures 412 and 414 to form the secondary winding W4 of FIGS. 1A-1B. The connection between the conductive structure 406 and the conductive structure 412 and between the conductive structure 414 and the conductive structure 412 is beneath the first terminal. The other portions of the conductive structures 406, 412, 414 and 418 include the terminals 420, 422, 424, 426, and 428 are each positioned above one of a series of second substrate structures 416. The second substrate structures 416 are individually patterned to have similar dimensions with terminals 420, 422, 424, 426, and 428. Moreover, the via structures 430 are configured to make connections between the terminals 420 and 424 and between the terminals 424 and 428.

In some embodiments, the first substrate 404 and second substrate 416 are separate substrates. In some embodiments, the first substrate 404 and second substrates 416 form a single substrate structure. In some embodiments, the first substrate 404 is a silicon (Si) substrate or a metal substrate. In some embodiments, the second substrate 416 is a Si substrate or a metal substrate.

In some embodiments, the via structures 430 and 432 are square vias. In some embodiments, the via structures 430 and 432 have octagonal shapes, hexagonal shapes, rectangular shapes, or the like. In some embodiments, the via structures 430 and 432 are through silicon vias. In some embodiments, the via structures 430 and 432 are holes etched in an interlayer dielectric that is filled with a metal. In some embodiments, the via structures 430 and 432 are buried vias. In some embodiments, the via structure 430 is different from via structure 432. In some embodiments, the via structures 430 and/or 432 are replaced with layered metal pairs to form interconnections with the conductive structures 406, 412, 414, and 418.

FIG. 5A is a schematic diagram of a transformer-coupled BPF 500, in accordance with some embodiments. The transformer-coupled BPF 500 includes a pair of coupled-transformers that is similar to the pair of coupled-transformers 400 of FIG. 4. Moreover, FIG. 5A includes the same coupling arrangement of the pair of coupled capacitive devices C_(c1) and C_(c2) as in FIG. 1A of the transformer-coupled BPF 106. The first terminal 434 is coupled to the voltage source V_(DD) and the input voltage (V_(G)) is connected to terminal 424. The coupled capacitive device C_(c1) is connected at one end to terminal 420 and the other end is connected to terminal 426. The coupled capacitive device C_(c2) is connected at one end to terminal 422 and the other end is connected to terminal 428. The transformer-coupled BPF 500 is an embodiment of the transformer-coupled BPF 106 of FIG. 1A.

FIG. 5B is a schematic diagram of a transformer-coupled BPF 502, in accordance with some embodiments. The transformer-coupled BPF 500 includes a pair of coupled-transformers that is similar to the pair of coupled-transformers 400 of FIG. 4. Moreover, FIG. 5B includes a similar coupling arrangement as in FIG. 1B of the pair of coupled capacitive devices C_(c1) and C_(c2) of the transformer-coupled BPF 112. The first terminal 434 is coupled to the voltage source V_(DD) and the input voltage (V_(G)) is connected to terminal 424. The coupled capacitive device C_(c1) is connected at one end to terminal 420 and the other end is connected to terminal 422. The coupled capacitive device C_(c2) is connected at one end to terminal 426 and the other end is connected to terminal 428. The transformer-coupled BPF 502 is an embodiment of the transformer-coupled BPF 112 of FIG. 1B.

The transformer-coupled BPFs 500 and 502 have similar properties described herein for the transformer-coupled BPFs 106 and 112 of FIGS. 1A and FIG. 1B. In particular, the transformer-coupled BPFs 500 and 502 have a transmission zero at twice the lower cutoff frequencies (2f₀). Moreover, the transformer-coupled BPFs 500 and 502 filters phase noise contributed by the 2^(nd) and 3^(rd) harmonics, as described herein.

FIG. 6A is a schematic diagram of a transformer-coupled BPF 600, in accordance with some embodiments. The transformer-coupled BPF 600 is similar to the transformer-coupled BPF 502 described in FIG. 5B. FIG. 6A includes the terminal 424 being coupled to the lowest metal layer 602, and the pair of capacitive devices C_(c1) and C_(c2) being similarly arranged as the transformer-coupled BPF 502 of FIG. 5B. The metal layer 602 is configured to be coupled to the voltage source (V_(G)). Also, the transformer-coupled BPF 600 is configured to provide isolation to prevent noise produced by parasitic resistance or the like. The width ratio between the width of the terminals (W₁) and the width of the metal layer 602 (W₂) is correlated to increasing isolation in the transformer-coupled BPF 604.

FIG. 6B is a graph of the isolation versus width ratio (W₁/W₂). The plot 604 includes two curves 606 and 608. The curve 606 includes the isolation versus width ratio for a transformer-coupled BPF operating at 28 GHz, and the curve 608 includes the isolation versus width ratio for a transformer-coupled BPF operating at 56 GHz. The higher the width ratio the better the isolation, as shown in both curves 606 and 608. One way to increase isolation is to decrease the width (W₂) of the metal layer 602. At 28 GHz and 56 GHz, a width ratio of 8 or higher is used.

FIG. 7 is a schematic diagram of a transformer-coupled bandpass filter (BPF) 700, in accordance with some embodiments. The transformer-coupled BPF 700 is similar to the transformer-coupled BPF 600 described in FIG. 6A. Also, FIG. 7 includes a pair of coupled capacitive devices C_(c1) and C_(c2) being positioned at a lower metal layer beneath the transformer-coupled BPF 700 at regions 714 and 716. The placement of the coupled pair of capacitive devices C_(c1) and C_(c2) at the lower metal layer beneath the transformer allows for sufficient separation between the coupled pair of capacitive devices C_(c1) and C_(c2) and the other operational elements of the transformer-coupled BPF 700. The separation lowers the effects of parasitic capacitance introduced by the coupled capacitive devices C_(c1) and C_(c2) to the remaining operational elements of the transformer-coupled BPF 700. Moreover, the space area of the transformer-coupled BPF 700 is decreased. In some embodiments, a layer of insulator material or the like is used to form the pair of coupled capacitive devices underneath the transformer.

FIG. 8A is a graph 800 of the output power of a VCO, in accordance with some embodiments. In particular, the graph 800 includes the output power versus frequency at the fundamental frequency and several harmonics of a first VCO according to another approach not having a transformer-coupled BPF as described herein and a second VCO according to the approach of a transformer-coupled BPF as described herein. The fundamental frequency is 28 GHz. At the second harmonic (56 GHz), the first VCO produces a drop of 22 dB in phase noise and the second VCO produces a larger drop of 28 dB. At the third harmonic (84 GHz) and fourth harmonic (112 GHz), the second VCO produces larger drops in phase noise relative to the first VCO due to filtering properties of the transformer-BPFs as described herein.

FIG. 8B is a graph of the noise suppression of a VCO, in accordance with some embodiments. In this case, FIG. 8B includes a graph 802 having two curves 804 and 806. The curve 804 is the simulated phase noise versus offset frequency of a first VCO according to another approach not having a transformer-coupled BPF as described herein and curve 806 is the simulated phase noise versus offset frequency of a second VCO having a transformer-coupled BPF as described herein. The phase noise of the second VCO is improved by 6.8 and 4.6 dB at 100 kHz and 1 MHz offset as shown in FIG. 8B.

FIG. 9A is a graph 902 of the measured frequency oscillation. The graph 902 includes two curves 904 and 906. The curve 904 includes a simulation of the oscillation frequency versus control voltage (Vctrl2) of a VCO having a transformer-coupled BPF as described herein, and the curve 906 shows the actual measurement of the oscillation frequency versus control voltage (Vctrl2) of the same VCO. The tuning range illustrated in plot 902 is between 27.2 GHz and 27.7 GHz. The more the control voltage increases the smaller the difference between the oscillation frequencies of the simulation and measurement becomes, as shown in FIG. 9A.

FIG. 9B is a graph 904 of the measured phase noise. The graph 904 includes two curves 908 and 910. The curve 908 is the measured phase noise at 27.4 GHz for a first VCO according to another approach not having a transformer-coupled BPF as described herein while curve 910 is the measured phase at 27.4 GHz for a second VCO having a transformer-coupled BPF as described herein. The measured phase noise for curve 910 is improved by 5 dB compared to the measured phase noise for curve 908. This improvement is consistent with the 4.8 dB phase noise reduction discussed in FIGS. 2A-2B.

FIG. 10 is a flowchart of a method 1000 of generating an oscillation signal, in accordance with one or more embodiments. The method 1000 is usable to generate a low phase noise oscillation signal. In step 1002, an input voltage, such as V_(G) (FIG. 2A), is received at an input terminal, such as terminal 424 of the pair of coupled-transformer 400, of a transformer-coupled BPF.

In step 1004, a power supply voltage, such as VDD, is received at a power supply input, such as first terminal 434 of the pair of coupled-transformer 400, of the transformer-coupled BPF.

In step 1006, the transformer-coupled BPF is coupled with a reference voltage, such as Vctrl2 or Vctrl1, through a cross-coupled transistor pair, such as M1 and M2. In some embodiments, the transformer-coupled BPF includes a pair of coupled-transformers having specific turn ratios. The transformer-coupled BPF includes a pair of coupled capacitive devices. In some embodiments, a first capacitive device of the pair of coupled capacitive devices is coupled at one end to a gate of a first transistor of the cross-coupled pair of transistors and the other end is coupled to a drain of a second transistor of the cross-coupled pair of transistors, and a second capacitive device of the pair of coupled capacitive devices is coupled at one end to the drain of the first transistor of the cross-coupled pair of transistors and the other end is coupled to the gate of the second transistor of the cross-coupled pair of transistors. In some embodiments, the first capacitive device of the pair of coupled devices is coupled at one end to a gate and the other end is coupled to a drain of a first transistor of the cross-coupled pair of transistors, and the second capacitive device of the pair of coupled capacitive devices is coupled at one end to a gate and the other end is coupled to a drain of a second transistor of the cross-coupled pair of transistors.

In step 1008, the oscillating signal is produced having a frequency that is synchronized with the frequency of the reference voltage.

An aspect of this description is related to a voltage-controlled oscillator (VCO) that includes a power supply node configured to have a power supply voltage. A reference node is configured to have a first reference voltage. A cross-coupled pair of transistors. A transformer-coupled band-pass filter (BPF) is coupled to the cross-coupled pair of transistors. The cross-coupled pair of transistors and the transformer-coupled band-pass filter are positioned between the power supply node and the reference node.

Another aspect of this description is related to a band-pass filter. The bandpass filter includes a first conductive structure in a first metal layer of an integrated circuit (IC). The first conductive structure includes a first terminal at a first end of the first conductive structure, and second and third terminals at a second end of the first conductive structure opposite the first end of the first conductive structure. A second conductive structure in the first metal layer, the second conductive structure includes an input terminal positioned between the second and third terminals at a first end of the second conductive structure, and a plurality of extending portions positioned at a second end of the second conductive structure opposite the first end of the second conductive structure. The extending portions extend from the input terminal to the first terminal in a direction opposite each other. A third conductive structure in the first metal layer, wherein a first end of the third conductive structure is connected to one of the plurality of extending portions of the second conductive structure, and a second end of the third conductive structure is positioned opposite the first end of the third conductive structure. The second end of the third conductive structure defines a fourth terminal. A fourth conductive structure in the first metal layer positioned opposite the third conductive structure, wherein a first end of the fourth conductive structure is connected to another of the plurality of extending portions of the second conductive structure, and a second end of the fourth conductive structure is positioned opposite the first end of the fourth conductive structure. The second end of the fourth conductive structure defines a fifth terminal. The second terminal is between the fourth terminal and the input terminal, and the third terminal is between the fifth terminal and the input terminal.

A further aspect of this description is related to a method of generating an oscillation signal. The method includes receiving an input voltage at an input terminal of a transformer-coupled band-pass filter (BPF). Also, the method includes receiving a power supply voltage at a power supply input of the transformer-coupled BPF. The transformer-coupled BPF is coupled with a reference voltage through a cross-coupled transistor pair. Furthermore, the method includes producing the oscillation signal whose frequency synchronizes with the frequency of the reference voltage.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

1. A voltage-controlled oscillator (VCO) comprising: a power supply node configured to have a power supply voltage; a reference node configured to have a first reference voltage; a cross-coupled pair of transistors; and a transformer-coupled band-pass filter (BPF) coupled to the cross-coupled pair of transistors, wherein the cross-coupled pair of transistors and the transformer-coupled band-pass filter are positioned between the power supply node and the reference node.
 2. The VCO of claim 1, wherein the transformer-coupled BPF comprises an input node configured to receive a second reference voltage different from the first reference voltage.
 3. The VCO of claim 1, wherein the transformer-coupled BPF comprises a pair of coupled-transformers having turn ratios of 1:2.
 4. The VCO of claim 1, wherein the transformer-coupled BPF comprises a pair of coupled capacitive devices.
 5. The VCO of claim 4, wherein a first capacitive device of the pair of coupled capacitive devices is coupled between a gate of a first transistor of the cross-coupled pair of transistors and a drain of a second transistor of the cross-coupled pair of transistors, and a second capacitive device of the pair of coupled capacitive devices is coupled between the drain of the first transistor of the cross-coupled pair of transistors and the gate of the second transistor of the cross-coupled pair of transistors.
 6. The VCO of claim 4, wherein a first capacitive device of the pair of coupled capacitive devices is coupled between a gate and a drain of a first transistor of the cross-coupled pair of transistors, and a second capacitive device of the pair of coupled capacitive devices is coupled between a gate and a drain of a second transistor of the cross-coupled pair of transistors.
 7. The VCO of claim 4, wherein the transformer-coupled BPF is configured to generate a transmission zero at a frequency substantially equal to twice a lower cutoff frequency of the transformer-coupled BPF.
 8. A band-pass filter (BPF) comprising: a first conductive structure in a first metal layer of an integrated circuit (IC), the first conductive structure comprising: a first terminal at a first end of the first conductive structure; and second and third terminals at a second end of the first conductive structure opposite the first end of the first conductive structure; a second conductive structure in the first metal layer, the second conductive structure comprising: an input terminal positioned between the second and third terminals at a first end of the second conductive structure; and a plurality of extending portions positioned at a second end of the second conductive structure opposite the first end of the second conductive structure , wherein the extending portions extend from the input terminal to the first terminal in a direction opposite each other; a third conductive structure in the first metal layer, wherein a first end of the third conductive structure is connected to one of the plurality of extending portions of the second conductive structure, and a second end of the third conductive structure is positioned opposite the first end of the third conductive structure, the second end of the third conductive structure defines a fourth terminal; and a fourth conductive structure in the first metal layer positioned opposite the third conductive structure, wherein a first end of the fourth conductive structure is connected to another of the plurality of extending portions of the second conductive structure, and a second end of the fourth conductive structure is positioned opposite the first end of the fourth conductive structure, the second end of the fourth conductive structure defines a fifth terminal, wherein the second terminal is between the fourth terminal and the input terminal, and the third terminal is between the fifth terminal and the input terminal.
 9. The BPF of claim 8 further comprising a plurality of vias configured to electrically connect the second conductive structure to the third conductive structure and the second conductive structure to the fourth conductive structure.
 10. The BPF of claim 8, wherein the first terminal is coupled to a power supply voltage source.
 11. The BPF of claim 8, wherein the input terminal is coupled to a second metal layer to reduce parasitic resistance.
 12. The BPF of claim 8, wherein the second terminal and the fifth terminal are each coupled to a first capacitive device.
 13. The BPF of claim 12, wherein the third terminal and the fourth terminal are each coupled to a second capacitive device.
 14. The BPF of claim 8, wherein the second terminal and the fourth terminal are each coupled to a first capacitive device.
 15. The BPF of claim 14, wherein the third terminal and the fourth terminal are each coupled to a second capacitive device.
 16. A method of generating an oscillation signal, the method comprising: receiving an input voltage at an input terminal of a transformer-coupled band-pass filter (BPF); receiving a power supply voltage at a power supply input of the transformer-coupled BPF; coupling the transformer-coupled BPF with a reference voltage through a cross-coupled transistor pair; and producing the oscillation signal whose frequency synchronizes with the frequency of the reference voltage.
 17. The method of claim 16, wherein the receiving the input voltage at the input terminal includes coupling the input voltage to a pair of coupled-transformers of the transformer-coupled BPF.
 18. The method of claim 17, wherein the receiving the input voltage at the input terminal includes coupling the pair of coupled-transformers to a pair of coupled capacitive devices.
 19. The method of claim 16, wherein the coupling the transformer-coupled BPF with the reference voltage includes operating the cross-coupled transistor pair in a saturation region for current stability.
 20. The method of claim 16, wherein the producing the oscillation signal includes reducing, by the transformer-coupled BPF, phase noise having frequency components greater than 2f₀, where f₀ is a lower cutoff frequency of the transformer-coupled BPF. 